Philosophy Compass is an excellent journal that publishes survey articles on the current state of the art on a given topic in a given sub-field of Philosophy, including topics in philosophy of religion. A number of very helpful survey articles in philosophy of religion have been published there recently, and I wanted to recommend some of them for those who can access them (if you can't, you might try Googling their department webpages for drafts):

When people think of Alvin Plantinga's work in the philosophy of religion, they typically think of his defense of the proper basicality of belief in God and his defenses of theism against the logical and evidential versions of the problem of evil. However, since about the early to mid 90s, he's gone on the offensive and offered several critiques of metaphysical naturalism.[1] His previous three arguments of this sort are:

(ii) The evolutionary argument against naturalism (EAAN): very roughly, the argument is that if naturalism is true, then the probability that unguided evolution gave us reliable cognitive faculties is low or inscrutable. Therefore, embracing naturalism gives one a decisive basis for not trusting your own mind.

(iii) The argument from proper function: very roughly, the argument is that a belief is warranted -- i.e., it has that quality or quantity, enough of which turns true belief into knowledge -- if and only if it's produced by truth-aimed cognitive faculties that are functioning properly in a congenial epistemic environment. But this analysis of warrant contains an irreducible appeal to proper function. But a naturalist can't make sense of the notion of proper function, as proper function in an entity x presupposes an intelligent agent who created the design plan for x, and from which x can diverge and malfunction. Therefore, the existence of proper function is evidence for supernaturalism.

Recently, Plantinga constructed a fourth argument of this sort. This one is aimed at refuting materialism in the field of the philosophy of mind. His current exposition of the argument can be found in his paper, "Against Materialism", Faith and Philosophy, 23:1 (January 2006), 3-32.

An interesting and powerful criticism of Plantinga's argument comes from his Notre Dame colleague and fellow Christian philosopher, Peter van Inwagen. Van Inwagen's critique is linked to there -- the paper with the title, "Plantinga's Replacement Argument".

================================Notes[1]The exception is the paper, mentioned in what follows above, on anti-realism, which is much older. On another note, I should say that I don't mean to imply that Plantinga has given no other arguments for theism or against naturalism. See, for example, his "Two Dozen (or So) Theistic Arguments".

Mike's argument seems to be this. The first three properties admit of an infinite number of degrees. Furthermore, at least very many combinations of these degreed properties are compossible -- perhaps infinitely many. Furthermore, these combinations are compatible with the fourth property of being concrete. So there seem to be an infinite number of compossible combinations of the first four properties:

GoodnessPowerKnowledgeBeing Concrete

Therefore, given the infinite number of possible combinations of these properties, it's highly probable that at least one such combination is compatible with the fifth property mentioned earlier, viz., the property of necessary existence. Therefore, since every set of compatible properties is possible, it's highly probable that a necessarily existent concrete individual with some degree of goodness, power, and knowledge is possible. And since (by Axiom S5 of S5 modal logic) whatever is possibly necessary is necessary simpliciter, it's highly probable that a necessary being with some degree of goodness, power, and knowledge exists.

In the comments of the post, Yujin Nagasawa raises the worry that the argument proves too much: if it really is true that at least one combination of the first four properties is compatible with necessary existence, then it also seems highly probable that other combinations of the properties are compatible with necessary existence as well. But if so, then the argument would seem to show that multiple necessary beings of this sort exist.

My own view is that our knowledge of what's metaphysically possible is quite limited, and thus that the probability of the compossibility of the first four properties with necessary existence is inscrutable. The basic idea is that while some properties are intrinsically compatible with other properties (e.g., the property of being a cat is compatible with the property of being white), some are not (e.g., the property of being a prime number is incompatible with the property of being a prime minister, to borrow an example from Plantinga). But if two or more properties are intrinsically incompatible with one another, then the fact that such properties come in degrees -- even infinitely many degrees -- is irrelevant to whether they can fit together coherently: if the properties involved are intrinsically incompatible, then not one of those combinations is possible. And the problem is that we don't know whether the properties involved in the argument are compatible (i.e., we don't know whether they are more like those involved in being a cat and being white, or more like those involved in being a prime number and being a prime minister).

...is now online at his department webpage at Notre Dame, here. (thanks, Al!). This 1983 text is the most developed exposition and defense of Plantinga's earlier internalist account of properly basic belief in God. It's still considered a classic. If you're interested in understanding and evaluating Plantinga's reformed epistemology, this is the place to start.

In his 1982 APA presidential address, "How to be an Anti-Realist", Alvin Plantinga (Notre Dame) argued that anti-realism is true, and that the best account of anti-realism entails the truth of theism. Plantinga summarizes his argument at the end of his address as follows:

"By way of conclusion then: the fundamental anti-realist intuition- that truth is not independent of mind--is indeed correct. This intuition is best accommodated by the theistic claim that necessarily, proposi- tions have two properties essentially: being conceived by God and being true if and only if believed by God. So how can we sensibly be anti- realists? Easily enough: by being theists."

Joe Salerno (Saint Louis University) and Berit Brogaard (U of MIssouri, St. Louis) published a paper ("Anti-realism, Theism and the Conditional Fallacy", Nous 39:1 (2005), pp. 123-139) that critiques Plantinga's and Rea's versions of the argument. The paper can be found here.

Richard Swinburne's The Existence of God (Oxford: Clarendon Press, 1979) is widely considered a high point in the defense of classical theism. There, he reformulated the classical arguments for theism as inductive arguments, using the machinery of Bayes' Theorem. He came out with a second edition of the book in 2004. A helpful review of the new edition can be found at Notre Dame Philosophical Reviews, here.

In his 2009 presidential address to the Society for Philosophy of Religion, Draper presents a paper arguing that theists bear a heavy burden of proof. His argument is based on an application of his new theory of intrinsic probability for the probability calculus. First, he briefly explains his theory of intrinsic probability, and why it's superior to Richard Swinburne's. He then applies his theory by showing resultant problems for generating good Bayesian arguments for theism. The audio of Draper's address can be found here.

"Few thinkers have had as much impact on contemporary philosophy as has Alvin Plantinga. The work of this quintessential analytic philosopher has in many respects set the tone for the debate in the fields of modal metaphysics and epistemology and he is arguably the most important philosopher of religion of our time. In this volume, a distinguished team of today’s leading philosophers address the central aspects of Plantinga’s philosophy - his views on natural theology; his responses to the problem of evil; his contributions to the field of modal metaphysics; the controversial evolutionary argument against naturalism; his model of epistemic warrant and his view of epistemic defeat; and his recent work on mind-body dualism. Also included is an appendix containing Plantinga’s often referred to, but previously unpublished, lecture notes entitled 'Two Dozen (or so) Theistic Arguments', with a substantial preface to the appendix written by Plantinga specifically for this volume."

Btw: the editor of the collection (Deane-Peter Baker) is a Christian philosopher who recently wrote a very helpful article on Plantinga's externalist account of warranted Christian belief ("Plantinga’s Reformed Epistemology: What’s the Question?", International Journal for Philosophy of Religion 57:2 (2005), pp. 77–103). After explicating Plantinga's account, he evaluates it by analyzing a virtually comprehensive list of criticisms of his account to date. He argues that the criticisms with real force reduce to four -- what he dubs (i) the proper basicality challenge, (ii) the ethical objection, (iii) the ‘further question’ objection, and (iv) the problem of religious diversity. He concludes that these criticisms have sufficient force to show that, pace Plantinga, positive arguments for Christian belief are required.